This invention relates to a porous composite membrane and to a process for making the porous composite membrane. More particularly, this invention relates to a porous composite membrane having a porous substrate and a modified surface wherein the composite membrane has essentially the same porous configuration as the porous substrate.
Presently, porous membranes are utilized for a variety of purposes including filtration. Membranes having a hydrophobic surface are utilized to filter compositions having a nonaqueous diluent or solubilizer liquid. Porous membranes having a hydrophilic surface are important in filtration applications which require the passage of water or aqueous process fluids through the membranes to remove unwanted materials in the aqueous fluids while avoiding the need for special wetting agents. It is also desirable that these membranes have good mechanical properties, good chemical resistance, good permeability and high retention. Hydrophilic membranes also may be charged (cationically or anionically) or neutral by controlling the chemical composition of the membrane substrate or by controlling the chemical composition of the superstrate or coating in the case of composite porous membranes. In any case, it is desirable that the entire surface of the composite membrane be modified with the desired surface characteristic and that the composite membrane have essentially the same porosity characteristics as the unmodified porous substrate. That is, the pores of the composite membrane should not be plugged so that the composite membrane retains its desired permeability.
Membranes having a neutral hydrophilic surface have no specific affinity for charged matter and therefore can be used in filtration applications such as is found in the pharmaceutical industry in which low binding of biological materials such as proteins, nucleic acids, or peptides by the membrane is desired. In contrast, charged membranes can be specifically modified to interact with dissolved and/or suspended charged matter to repel or bind materials having the same or opposite charge as the membrane. In filtration applications, cationically charged membranes having a hydrophilic surface are capable of removing unwanted negatively charged materials of smaller dimensions than the rated pore size of the membrane through charge interactions rather than by typical sieving mechanisms. This characteristic is particularly useful in the production of high purity water used in the microelectronics industry where maximum particle retention as well as high flux are desired. Membranes having a cationically charged surface also are used in the biotechnology industry where membranes are used to bind and immobilize biomolecules such as nucleic acids, peptides, proteins or the like in blotting applications. Anionically charged membranes may be used in a fashion similar to cationically charged membranes or as ion exchange materials for electrolytic applications.
It is desirable to provide membranes having a modified surface from which extractables are minimized or eliminated. By preventing the introduction of extractables from the membrane into the fluid being passed through the membrane, purity of the fluid is maintained.
It has been proposed in U.S. Pat. No. 4,618,533 to form a composite membrane by coating a porous membrane substrate with a polymerizable monomer, a free radical initiator and a crosslinking agent. The monomer is polymerized and cross-linked in situ to form a cross-linked second polymer as a superstrate which can comprise a hydrophilic coating. The composite membrane thus formed has essentially the same porous configuration as the porous membrane substrate. Over time, the cross-link bonds may become degraded which can result in the loss of the coating or superstrate and the formation of molecular species that are extractable into fluids being processed through the composite membrane.
U.S. Pat. No. 4,113,912 discloses a composite porous structure having a hydrophilic surface comprising a porous fluorocarbon resin substrate with pores containing at least one water soluble polymer which has been water-insolubilized. Suitable water soluble polymers that are water insolubilized include polyvinyl alcohol, polyethylene oxide or polyacrylic acid. The water soluble polymer is rendered water insoluble by a heat treatment in the absence of a polymerization initiator, chemical reaction with crosslinking agents resulting in acetalization or esterification, chemical reaction with potassium bichromates as crosslinking agents or by cross linking with ionizing radiation but in the absence of a polymerization initiator. Suitable heat treatments disclosed are 150.degree.-160.degree. C. for about 4-6 minutes or 200.degree. C. for about 1 minute. The use of these high temperatures, while necessary to effect crosslinking are undesirable since they can cause degradation of the substrate, particularly low melting substrates such as polyethylene. In addition, the use of ionizing radiating can cause substrate degradation. The use of the chemical crosslinking reagents also may result in bonds which become degraded thereby leading to undesirable extractables and loss of the coating or the superstrate.
It has also been disclosed by Sanderson in Desalination, 90 (1993) page 15-29 that an insoluble nonporous base layer of a reverse osmosis membrane which has little or no porosity can be formed from water insolubilized polyvinyl alcohol.
Accordingly, it would be desirable to provide a composite porous membrane having a small average pore size and which has a completely modified surface which can be hydrophilic or hydrophobic and which retains substantially the same porous configuration as the porous substrate. In addition, it would be desirable to provide such a membrane while avoiding degradation of the membrane substrate and the production of extractables.